Discovery and development of new materials for ambient temperature sodium batteries are increasing, with the goal of providing energy storage to, for example, grid applications throughout the world. Generation of energy arising from renewable resources, such as conversion of wind, solar or wave sources must be stored in batteries for later use. There are a number of storage technologies that dominate the present market, such as large Li-ion batteries, aqueous Na batteries, high temperature liquid Na based batteries, and others, but the lifetime, safety, up front capital costs, and too low energy densities plague these technologies. In addition, power pulse characteristics are critically important to negotiate generation peak fluctuations in the grid. Many of these technologies also fall short of this demand.
Cost is an issue when large-scale grid storage systems are built, because of their massive size (MWh levels), therefore one would like to replace Li-ion based batteries since they are expensive, are subject to price fluctuations in the market (due to transportation, and consumer applications), and lithium reserves may be limited. On the other hand sodium-based batteries clearly will have the promise of low-cost due to abundant Na precursors used to make electrodes. Furthermore, iron-based electrode materials, discussed in this background also consist of Earth-abundant elements for the application.
However, sodium-based batteries have the disadvantage that there are limited electrode materials capable of inserting Na reversibly due to the large size of the atom and it's cation (for charge storage). In this regard, it is possible to employ nanomaterials that can use their high-surface area for concentration of sodium cations at the surface, and their defect vacancies in the structure that will enable the Na cations to insert into such vacancies. Bulk materials, in contrast, typically do not have such defects and vacancies.
Controlled electrode architectures that are formed with nanostructures are increasingly being used as electrodes for batteries, as they provide good high-power pulses, long life and decent energy densities. Many nanostructured materials are better than ultracapacitors or pseudocapacitors in terms of energy density, but have higher power characteristics greater than conventional intercalation laminate-type electrodes in batteries.
For materials with nanosized dimensions, the surface area is increased, and thus the importance of interfaces and surfaces is elevated. Material properties like voltage profiles, charge-discharge curves, and also phase diagrams can be affected. Improved storage properties, and materials that are inactive at the micro size can become excellent storage materials when nanosized. In this context, therefore, novel surface storage mechanisms might explain these phenomena. Phase transitions, such as those that are first-order, and which are responsible for the constant voltage output of batteries, can be suppressed at the nanoscale. Since the phase transitions may be muddled, the morphological transitions or transformations of the actual nanoparticle can affect the chemistry. However, the speed of cation diffusion along the surface of the nanoparticle is known to be extremely fast due to the short nanosized dimension and curved surfaces. This can allow large cations to reach intercalation sites, particularly defect vacancies with fast charge transfer throughout the particle. The availability of interstitial sites for the cations can be a general problem in dense lattices that are fully filled. As a result, the capacities and associated energy will be lower and may mimic micron sized particles. The power may be increased, but the high surface areas also can lead to unwanted side reactions. A balance is needed between all of these properties.
In Li+ ion batteries, anodes made from solid nanoparticles and microparticles, as well as hollow nanostructures (e.g., TiO2, Co3O4, SnO2, CuO, and α-Fe2O3) have been evaluated. Also, hollow structures of lithiated layered oxides such as LiNi0.5Mn1.5O4 and LiMn2O4 have been used as cathode materials.
New strategies for replacing lithium ion batteries with energy storage systems based on more Earth-abundant elements are needed because of limited lithium sources. Sodium ion batteries are promising candidates for large-scale energy storage systems owing to the abundance and low cost of sodium. However, it has been difficult to find appropriate Na electrode materials since the large size of the sodium ion often leads to rather limited intercalation or extrusion into and out of active electrode materials. As a result, materials that efficiently work with lithium ions often either do not work at all of sodium ions, or show very limited activity with sodium ions. The use of layered and tunnel-type oxides and phosphates have been proposed for batteries based on Na ions to address the problems associated with sodium ion batteries. Bulk materials (i.e., relatively large particle size materials) have demonstrated relatively low capacity and cyclability (<130 mAh/g at rate slower than 0.1 C), compared to nanostructured materials, which have been reported to produce higher capacity and better cyclability. For example, Cao et al. (J. Phys. Chem. C, 2008; 112: 1851) reported a capacity of about 128 mAh/g with 77% capacity retention after 1000 slow rate (0.5 C) cycles for a Na ion cathode made from manganese oxide nanowires. Nanostructured bilayered vanadium oxide-based cathodes reportedly demonstrated reversible (250 mAh/g) capacity at slow rate (20 mA/g, 0.125 C) cycling with Na+ ions up to 350 cycles (Tepavcevic et al. ACS Nano, 2012; 6: 530). Also, amorphous TiO2 nanotubes reportedly have exhibited promising performance cycled at a high rate as an anode for Na+ ion batteries (Xiong et al., J. Phys. Chem. Lett., 2011; 2: 2560).
There is an ongoing need for new electrode materials for sodium ion electrochemical cells and batteries, as alternatives to lithium ion-based materials. The electrodes, electrochemical cells, and batteries described herein address this need.